Vertical velocity
by
staalstrom
—
last modified
Aug 30, 2006 10:59 AM
Hallo Atle
This is Andre Staalstrøm working in PGS.
I have seen this positive vertical velocity on two different VMDP systems on two diffrent ships, and it has pusseled me. But I've just assumed that it wasn't a problem, and I'm glad to see that that was right.
But now I want to calculate the the Reynold Stress in the water, and then I need a correct vertical velocity.
So how do I correct it? What exactly is tilted 10°? The instrument itself, or the beam?
Best regards
André
This is Andre Staalstrøm working in PGS.
I have seen this positive vertical velocity on two different VMDP systems on two diffrent ships, and it has pusseled me. But I've just assumed that it wasn't a problem, and I'm glad to see that that was right.
But now I want to calculate the the Reynold Stress in the water, and then I need a correct vertical velocity.
So how do I correct it? What exactly is tilted 10°? The instrument itself, or the beam?
Best regards
André
Current state:
Being created
Dear André
Sorry for the late answer -- vacation time has taken its toll on the forum response rate this summer.
The issue of vertical velocities under a ship is a very interesting question and not one that is easily answered
Several researchers have studied the problem in detail, both using current profilers and with ship modelling studies. From everything I have seen, it clear that this "apparent" vertical velocity is generated by the ship itself. In other words, the water dips down at the bow and comes back up at the stern. Pretty intuitive when you think about it.
What is less intuitive is the depth to which the ship generated vertical velocity can reach. For large research ships I have seen traces of vertical velocity as far down as 50-m - but this obviously depends on the size of the ship (and to some degree also the speed).
The impact on the horizontal velocity is been quite small and for the purpose of streamer handling )with low ship speed) it can be assume to be irrelevant. For scientific purposes, however, it can be of significance when looking at data very close to the ship.
When it comes to calculating Reynolds stress, I must admit I would not even try. It is sort of inconceivable that it could work out with all the uncertainties you have in the tilt angles, the relative high noise in the 1-s data, etc. But it is not for me to stop anyone from attempting
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Best regards, Atle Lohrmann
Sorry for the late answer -- vacation time has taken its toll on the forum response rate this summer.
The issue of vertical velocities under a ship is a very interesting question and not one that is easily answered
Several researchers have studied the problem in detail, both using current profilers and with ship modelling studies. From everything I have seen, it clear that this "apparent" vertical velocity is generated by the ship itself. In other words, the water dips down at the bow and comes back up at the stern. Pretty intuitive when you think about it.
What is less intuitive is the depth to which the ship generated vertical velocity can reach. For large research ships I have seen traces of vertical velocity as far down as 50-m - but this obviously depends on the size of the ship (and to some degree also the speed).
The impact on the horizontal velocity is been quite small and for the purpose of streamer handling )with low ship speed) it can be assume to be irrelevant. For scientific purposes, however, it can be of significance when looking at data very close to the ship.
When it comes to calculating Reynolds stress, I must admit I would not even try. It is sort of inconceivable that it could work out with all the uncertainties you have in the tilt angles, the relative high noise in the 1-s data, etc. But it is not for me to stop anyone from attempting
" />'> Best regards, Atle Lohrmann
Current state:
Being created
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